Battery saving circuits are used to minimize power consumption by periodically, rather than continuously, supplying power to a receiver. Presently known circuits operating in radio receivers periodically supply power, search for the presence of an RF (radio frequency) carrier and then, if a carrier is found, extend the time the power is supplied to permit a further search for a valid coded squelch signal (CSS). Such squelch operated battery savers have a significant disadvantage in that every receiver within a system is activated whenever any transmission of a carrier signal occurs, regardless of which individual receiver is intended to be reached. Typically the CSS detect time in many systems ranges from 110 to 170 msec. minimum with the presence of a strong signal condition. In these systems the receiver dead time or access delay is the sleep time plus receiver warm up and the carrier detect for a no carrier (inactive channel) condition. This dead time is increased by the time it takes to detect the CSS when there is activity on the channel. It is obvious that in busy systems these timing requirements put a major burden on the effectiveness of a squelch operated battery saver to the point where they are always up and checking the air waves either for carrier or CSS.
One solution to reduce the detection time in a squelch operated battery saver is to check the incoming CSS only once for a detected carrier. Thereafter the receiver checks for the presence of the carrier only and assume that the same CSS is being received for the duration of the detected carrier. This solution reduces the detection time significantly since the carrier detect time (6 to 15 ms) is considerably shorter than the CSS detect time. However such a solution introduces a new problem in situations where a second transmitter transmits a signal, perhaps an emergency signal, intended for a receiver that is in the sleep mode. The receiver wakes up periodically and determines that the carrier is still present and erroneously assumes that the CSS has not changed returning to its sleep mode. Such assumption results in the receiver not checking the received CSS which now contains the right code for this receiver further resulting in the missing of the signal from the second transmitter. This deficiency has rendered such a solution less desirable. It is therefore clear that a need exists for a battery saver method to effectively put a receiver in a sleep mode even in very busy systems without the loss of any incoming signals.